from finking below the top of the ftem. The ftem A B, muft be fol- STATICS. der'd on to the ball, at the upper hole thereof. It may be either hollow, or folid; but it ought to be made very flender, and confiderably long. In the lower hole of the ball, is folder'd the undermoft part of the inftrument, which I call the ftirrup G. The fcrew F, is a very fhort Fig. 28. piece of brafs, with a broad flit in it, capable of receiving the edge of the coin; which, with a turn or two of a flight fmall fide-fcrew, may be kept faft in. The ftirrup G, is made of wire, which, a little below the bottom of the ball, is bent round, fo as to ftand horizontally, that the coin may be laid upon it. "Twould be convenient to have the under ftem and fcrew made separate, that the ball of the inftrument, being fufficiently large, there may be room for two or three flat round pieces of metal K, with each of them a hole in the middle, fitted to the fize of the Fig 29. ftem, to serve as balaft; being placed near the lower part of the ball, when the fcrew may be thruft on to fupport them.

To adjust this inftrument for the examination of a guinea, you must, Applied to the by the help of the ftirrup, or fcrew, hang a guinea, known to be examination of genu- coin. ine, at the bottom of it; and having ftop'd the orifice of the ftem, if hollow, that no water may get in, immerfe the inftrument gently, and perpendicularly in a veffel of fair water, almoft to the top of the ftem; then letting it alone, if, being fettled, it continues in the fame ftation, and pofture, the work is done. If it emerge, add a little weight to it, either at the bottom, or the top, that the guinea may deprefs it as low as is required; but if it fink quite under the water, lighten it, by filing off from the balaft-plates, above-mentioned, or otherwife. Then fet a mark H, Fig. 27. where the furface of the water touches the ftem; and taking out the inftrument, fubftitute in the place of the guinea, a little round plate of brafs, of the fame weight, or a grain or two heavier, in the air; and putting the inftrument into water, as before, let it fettle, and make another mark I, at the interfection of the stem, and the horizontal furface of the water.

But there may happen a cafe, wherein the practical application of our inftrument will he unfecure. For, if a falfifier of money have the skill, by wafhing, or otherwife, to take off much of the fubftance of the coin, without altering or impairing either the figure, or the ftamp; the coin wilk not deprefs the inftrument to the ufual mark, whence the guinea may be judg'd counterfeit, tho' it be only too light. However, it prefently fhews, that the propofed guinea is abused; and, if the want of weight appear, by the inftrument, to be very great, 'tis a ftrong prefumption, that it is rather wafh'd, than counterfeit. But the balance will prefently refolve the doubt : for, if the fufpected coin has its due weight in the air, the great lightness of it in water, feems to proceed from its not being of the requifite finenefs; and, if it want much of its due weight in the air, 'tis probably wafh'd, &c. rather than adulterated.

It may be very convenient, for those who have frequent occafions to examine various forts of coin, to have a different inftrument adjusted to each. But if the ball be made large, and with a ftem long and flender enough, we

may

STATICS. may foon, by changing the balaft-plates occasionally, fit the fame inftrument to examine coins of different metals, and weights. Thus one of these, made of copper, ferves me for guineas, crown-pieces, and half-crowns; and may eafily be made to ferve for several foreign coins.

And other mix'd metals.

After the fame manner, may pewter be compared and examined. For, having once obferved, how far the inftrument is funk, by a determinate weight of it, which is known to be good in its kind; and to contain its due proportion of lead and tin; if you load the inftrument with an equal weight of any other mafs of pewter, and it fink deeper, the proportion of lead, very probably, exceeds in the mixture; tho' 'tis poffible to debafe pewter with mineral fubftances, whofe fpecific gravities are not well known: but to do it with lead, feems to be the moft gainful way.

This inftrument, may, alfo, affift us to make a near eftimate of the fineness of gold, and its different alloys with filver, or other determinate metals: for, being fitted to fink to the top of the pipe, with a certain weight of the pureft gold, as fuppofe an ounce, make a mixture containing a known proportion of the metal wherewith you alloy the gold, fuppofe, of nineteen, or fifteen parts of gold, and one of filver; and letting the inftrument fettle in the water, mark the place where the furface of the fluid cuts the stem. Then putting in another mixture, wherein the filver bears a greater proportion to the gold, as, fuppofe the former be an eighteenth or a fourteenth part of the latter, you may obferve how much lefs than before, this depreffes the inftrument: and fo proceed to as many degrees of alloy, as can be diftinguished on the ftem; always obferving, that the weight of each mafs, in the air, be exactly the fame with that of the pure gold; which we suppose an

ounce.

By the fame method may be examined the different alloys of pure filver, with copper, or other metal, fpecifically lighter than filver; and, with a flight variation, 'twill not be difficult to eftimate in what degree, feveral coins, whether filver, or gold, are debafed, by the known ignobler metal mixed in the piece propofed; and this with greater exactnefs, than by the touch-ftone, or much more troublesome and chargeable methods. And the fame is applicable to other mixtures of metals.

A

A

Statical Hygroscope.

H

SECT. I.

AVING had occafion, amongst other fubjects relating to the air, Various bodies prepofed, to difto confider its moisture, and drynefs, I eafily difcern'd, they had cover the dryness no fmall influence upon feveral bodies; and, among the reft, upon and and moisture of thofe of men; as the air we breathe, either paffes from one of these fluid qualities to the other; or from one degree to another, in the same quality.

I, therefore, immediately confider'd of a proper way, to discover the changes of the air, as to moisture and drynefs; with their feveral degrees. For this purpose it seem'd, to me, that, if a ftatical hygrofcope could be contrived, it would be very convenient; and fit, both to determine the degrees of the moisture, or drynefs of the air; and to tranfmit the obfervations made of them to others. And confidering, further, that among bodies, otherwife well qualified for fuch a purpofe, that was likelieft to give the most fenfible informations of the changes of the air, which, in refpect of its bulk, had the largeft furface expofed thereto; I pitch'd upon a fine fpunge, as a thing which is portable, not eafily diffipated, but, by its readinefs to imbibe water, feem'd likely to attract the aqueous particles it might meet with in the air, and which, by its great porosity, has much more of fuperficies, with regard to its bulk, than any body, not otherwise less fit for the purpose.

But I here propofe, not the beft hygrofcope that might be contriv'd, only one that is fimple, cheap, contained and fet up in a little room, eafy to be made and tranfported, ftatical, commodious, and applicable to various ufes.

In confidering, what bodies were the fitteft to be employ'd for ftatical hygrofcopes, I made tryal of more than one, that feem'd not the leaft promifing. I know, that common, or fea-falt, will greatly relent in moift air; and falt of tartar much more: but then, these falts will not eafily part with the aqueous corpufcles they have once imbibed; and are, in other respects, lefs convenient than a fpunge. I made tryal, alfo, with lute

ftrings,

the air.

STATICS. ftrings, chofen very flender, that they might have the greater furface, in refpect of their bulk; and thefe I found very well to imbibe the moifture of the air: but, afterwards, they did not anfwer my expectation. I, alfo, caufed a cup to be turn'd out of light wood, which, that it might lefs burden a tender balance, had, inftead of a foot, a little button, whereto a hair might be tied, to fufpend it; and this cup, being exceeding thin, that it might have much furface expofed to the air, proved fo good a hygrofcope, that I made feveral obfervations with it, for a confiderable time. It agreed, alfo, with many trials, made on other occafions, that white fheeps leather, fuch as is ufed for plaifters, would be very convenient for my purpose. And, indeed, I found, by feveral obfervations, that if this leather were a fubftance as little liable to corruption as a spunge, it would be a fitter matter, than any other I had employ'd, for hygrofcopes.

A Spunge made choice of.

And turn'd into

But, upon the whole, I found no body fo convenient for my purpose, as a fpunge to try the porofity whereof, we weigh'd out a dram of the fine fort; and having fuffer'd it to foak up what water it could, we held it in the air, till it dropp'd fo very flowly, that we counted a hundred between the falling of two fucceeding drops; then putting it into the balance wherein it had been weigh'd before, we found, that as its dimensions were increased to the eye, fo its weight increased upon the scale; amounting now, to above two ounces and two drams; fo that one dram of fpunge, tho' it feem'd not altogether fo fine as the portion we had chofen for our hygrofcope, imbibed and retained feventeen times its weight of

water.

Now a fpunge is eafily turn'd into a hygrofcope; for, having weigh'd abygrofcope. it when the air is of a moderate temperature, it only requires to be put into the fcale of a good balance, fufpended on a fix'd and itable fupporter. For the fpunge, being carefully counterpoised, at firft, with a metalline weight, which alters not, fenfibly, with the changes of the air, it will, by its rifing or falling, fhew how much the neighbouring air is grown dryer or moifter, in the place where the inftrument is kept. The weight of the fpunge may be greater, or lefs, according to the fize and goodness of the balance, and the accuracy required in the difcovery. For my part, tho' I have, with very tender fcales, long employed but half a dram of fpunge, I found it anfwer my expectation: and tho', when I ufed a bulk feveral times as great, in a ftronger, but proportionably lefs accurate balance, I had ill fuccefs; yet, after trials with different quantities of it, I preferr'd that of a dram; as not being fo heavy as to overload the finer fort of goldfmith's fcales, and yet fufficient to discover changes confiderably minute; turning, difcernably, with a fixteenth or twentieth part, and, manifeftly, with half a quarter of a grain.

Experiments

With fuch hygrofcopes as thefe (wherein the balance ought to be ftill made with it kept fufpended and charged) I made feveral trials, fometimes in the fpring, fometimes in the autumn, and fometimes, alfo, in the fummer and winter. Having once kept a dram of fpunge fufpended, during a

whole

whole fpring, and a great part of the preceding winter, and fubfequent STATICS. fummer, I was well rewarded by the obfervations it afforded me.

Now, the use of a hygrofcope is either general, or particular.

The general ufe is, to estimate the changes of the air, as to moisture The use of hy and drynefs, by fuch ways of measuring them, as are eafy to be known, grofcopes. provided, and communicated. 'Tis obvious that our ftatical hygroscope has the advantage of the common forts, in thefe feveral respects.

There is, however, one great imperfection, which all that I have been acquainted with, are liable to; the want of a standard of the drynefs and moisture of the air; by relation to which, hygrometers should at first be adjufted, and afterwards compared with one another: tho' this inconvenience may poffibly be remedied. In the mean time, even our flight inftrument will, in many respects, prove very serviceable.

the statical hy

riations of wea

hour.

And firft, it will, on many occafions, be useful to fhew, both that the The first use of air is ufually lefs moift at one part of the artificial day, than at any other; grofcope, to how and at what particular time of the day, or night, it most commonly is fo. the different vaI remember, when the weather was at a ftand, I usually obferved, that ther in the fame the spunge had manifeftly gain'd in the night, tho' it were kept in a bed-month, day, and chamber, and grew lighter again between the morning and noon. But this obfervation, which was made towards the end of winter, would not hold, in cafe frofty nights, or fome other powerful caufe intervened. 'Twere proper, alfo, to obferve, whether there be not a correfpondence betwixt the hygrometer, and barometer,; and, if there be, in what kind of weather, or conftitution of air, it is moft, or leaft to be difcerned. And this enquiry feems the more dubious, because the fame changes of the atmosphere may, upon differing accounts, have either the like, or quite contrary operations upon these two inftruments. For, in fummer, when the atmosphere is ufually heavier, the hygrofcope is ufually lighter: fome ftrong winds, as with us the north-weft, may make both the atmosphere, and barometer lighter; whereas, foutherly winds, especially if accompanied with rain, often make the atmosphere lighter, and the fpunge heavier. On the other fide, I obferve, that eafterly winds, especially when they begin to blow in winter; tho', by reason of their drynefs, they make the hygrofcope lighter; yet, at leaft here, at the weft-end of London, they commonly fhew the air, by the barometer, to be heavier. It were, likewise, fit to be obferved, particularly by thofe who live on the fea-coaft, whether the daily ebbing, or flowing of the fea, do not fenfibly alter the weight of the hygrofcope. Twould, alfo, be very convenient, to take notice, at what times of the day or night, cateris paribus, the air is moft damp and moft dry; and not only in feveral parts of the fame day, but in feveral days of the fame month; efpecially on thofe, wherein the full or new moon happens. And this feems a more hopeful way of difcovering, whether the full moon diffufes a moisture in the air, than thofe vulgar traditions of the plumpness of oyfters, shell-fish, the brains of fome animals, the marrow in their bones, c. It may alfo be noted, whether monthly fpring-tides, efpecially when Ccc they

VOL. II.